Manufacture of alumina.



' To all whom it may concern:

, UNITED STATES PATENT' OFFICE. GILBERT McoULLooH, OF nAs'r ST.- LOUIS, ILLINOIS,- AssIGNoR'ro ALUMINUM coM- PANY or AMERICA, or nIAGARA FALLS, NEW YORK, A CORPORATION OF PENN- SYLVANIA.

MANUFACTURE OF ALUMINA.

941, 799, Specification of Letters Patent.

No Drawing.

Be it known that I, GILBERT MCCULLOCH, of East St. Louis, county of St. Clair, and State of Illinois, have invented a new and useful Improvement in the Manufacture of Alumina, of which the following is a specification.

My invention is an improvement in the manufacture of almnina, and is applicable to any processof manufacture in which the alumina is precipitated from a solution of sodium aluminate by means of carbon dioxid gas. It is applicable, for example, to the process described in the patent of Charles M. Hall, No. 663,167, dated December 4, 1900. In the practice of such process a solution of sodium aluminate is formed, which may be done by any of the methods known to the art, and alumina is precipitated from it by passing over it or through it gas containing carbon dioxid. In the resultng reaction the carbon dioxid combines with the soda to ite carries a considerable proportion of silica which 1n the ordinary processes of prec1p1 ta-* tion by means of carbon dioxid is precipitated along With the alumina and thus contaminates the product. I have discovered that in the process of precipitation the alumina is precipitated first and that the silica is mainly precipitated at the end of the operation; and have discovered that by arresting the precipitation before it is completed the bulk of the silica may be retained in solution along with a portion of the alumina and the sodium carbonate and may be run back to the digesters for treatment with additional bauxite and caustic lime, as described, for example, in the patent of Charles H. Hall, No. 663,167, to form a new solution of sodium aluminate. The silica which is thus returned with the sodium carbonate liquor to the digesters does not aecumulate in the liquor, but acts as the silica which is introduced as an impurity with the bauxite. The great bulk of the silica present, namely, that which is in the solution and that which is introduced with the baux- Patented Nov; 30, 1909.

Application filed April 31), 1909. Serial No. 493,076.

ite, remains in undissolved form with the undissolved portions of the bauxite and calcium carbonate, if lime is used, and. constitutes what is known in the art as red mud.

The following is an example of the practice of my invention in the form which, I prefer. In carrying out my'precess I take a liquor of approximately the following composition: A1 0,, 75 to 80 grams per liter; NA GO 40 to 50 grams per liter; Na. combined. with Al,(),, and reckoned as NaOH, 105 to 115 grams per liter; Si(),, .15 to grams per liter. I then charge this liquor into an agitator, which is preferably a long steel cylindrical tank. In this agitator I keep the liquor well stirred by paddles and pass over it- CO, gas, preferably from a lime kiln, which contains usually 23 or 24 per cent .-.of C The Cl) gas re- -ac.ts with the sodium aluminate, forming Na CO and aluminum hydrate until the aluminapresent in solution has been reduced to 5 to grams per liter. The composition of the liquor is then about as follows: A1 0 5 to grams per liter; Naj), combined with Al,(),,, and reckoned as NaOll, 10 to grams per 'liter; Si(),, .10 to .20 grams per liter, Na. .CO,;, 185 to 195 grams per liter.

I have found that as the alumina is precipitated out of the liquor, by the action of the CO 'as, the silica is also precipitated, but not'in the same, ratio, as the alumina. The following table shows approximately the ratio atwhich the alumina and silica precipitate out where the original solution contains approximately .25 grams of silica per liter.

When of alumina has been precipitated, 4% ofbsilica has &

een precipi tated. in u i. t. t gg; .355 \L at u \l ti \i B u H H h LL Li I h u H (I M H 'dosired from the silica standpoint.

The above simply serves asan example, as with solutions containing more silica the precipitation would be stopped earlier, and with solutions containing less silica the preportion of alumina may cipitation may be continued until a larger proportion of alumina is precipitated.

Instead of filtering the aluminum hydrate and running the liquor back to the digesters to take up a new charge of alumina from bauxite, the liquor containing a small proafter filteration be run back to the agitators or tors and the precipitation completed by means of ()0 gas, giving a quantity of an inferior grade of aluminum hydrate which would be useful for some purposes.

I claim:

1. In the process of manufacture of a1u-,

mina by the alkaline process Wherein the alumina is precipitated from a'solut-ion of mullllln aluminate by C0 gas, the improvemcnt which consists in arresting the precipitation after a large proportion of the alumina. has been precipitated from the solution, whereby a large proportion of the contained silica is retained in the solution.

2. In the process of manufacture of aluinina by the alkaline sodium alummate by C0 gas, the improvement which consists in arresting the preto other agitaprocess wherein the alumina is precipitated from a solution of cipitation after a large proportion of the alumina has been precipitated from the solution whereby a large proportion. of the contained silica is retained in the solution,

and Withdrawing the liquor containing the residual silica and alumina and using it as a basis for dissolving other alumina from' bauxite.

3. In the process of manufacture of alumina by the alkaline process wherein the alumina is precipitated from a solution of sodium aluminate by C0 gas, the improvement which consists in arresting the preci itation after a large proportion of the al nlina has been precipitated from the solution andbefore the precipitation of a considerable proportion of the contained silica and thereafter drawing off the solution-and completing the precipitation of the alumina by additions of CO gas. I

In testimony Whereof,-I have hereunto set my hand.

GILBERT MoCULLOCH. 

